Compressor Overheating – Short #99

In this short podcast episode, Bryan explains what happens to a compressor when it's overheating. He also covers possible causes and troubleshooting strategies.

One of the Kalos techs came across an overheating compressor case that looked like a textbook TXV problem: the superheat was high at the condensing unit on the compressor side. However, the air handler superheat was appropriate, and the suction pressure was low. TXVs, however, respond to the superheat dropping and reduce the pressure even more. Overall, the mass flow rate and velocity drop, meaning that the refrigerant temperature can increase as it spends more time in the suction line.

We were missing a few key measurements to diagnosing compressor overheating. In those cases, we want to know the return gas temperature, discharge line temperature 6 inches out from the compressor, and the compression ratio (absolute discharge pressure / absolute suction pressure). You'll generally want to see a compression ratio between 2.6 and 3 on residential HVAC equipment; the lower the compression ratio, the better the efficiency. A compression ratio higher than 3 can lead to compressor overheating. A return gas temperature consistently above 65 degrees can also make a compressor run hot. The discharge line temperature should not exceed 225 degrees.

Then, you must determine if the charge is correct. (Are you starving the evaporator?) Check if you have restrictions and if your suction line is improperly insulated. Restrictions and heat transfer in the suction line can lead to compressor overheating.

It's bad for a compressor to run hot, but they can go their entire lives without tripping on the thermal limit. Compressors that run hot can have lubrication issues and will have shorter lifespans. The best thing you can do is try to reduce the compression ratio. (Clean the condenser, keep head pressure low, keep good indoor airflow, etc.)

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